1. Field of the Invention
The present invention relates to ceramic laminate bodies for use in stack type gas sensor elements and related manufacturing methods and, more particularly, to a ceramic laminate body, composed of a stack of plural ceramic sheets, and a method of manufacturing the ceramic laminate body.
2. Description of the Related Art
Various attempts have heretofore been made to provide laminate type gas sensor elements each of which includes a structure composed of a stack of plural ceramic sheets. The adjacent ceramic sheets are stacked on each other to form a closed hollow space for introducing atmospheric air serving as reference gas or measuring gas.
Such a structure of the ceramic laminate body is disclosed in FIG. 16. As shown in FIG. 16, the ceramic laminate body 110 comprises two ceramic sheets 112, 114 that are stacked on each other via an intermediate bonding layer 116 so as to form a closed hollow space 118 between the two ceramic sheets 112, 114 as disclosed in Japanese Unexamined Patent Application Publication Nos. 9-304321 and 2001-30219.
In manufacturing the ceramic laminate body 110, for instance, an intermediate bonding layer paste is partially coated on one of ceramic green sheets and the other one of the ceramic green sheets is stacked on the one of the ceramic green sheets via the intermediate bonding layer paste, thereby forming an unburned laminate body. Subsequently, the unburned laminate body is fired, thereby obtaining the ceramic laminate body 110 having the closed hollow space 118.
However, during step of firing the unburned laminate body, a stress had occurred in a boundary layer between the ceramic green sheet 112 and the bonding layer paste 116 as shown in FIG. 17, causing a cracking 99 or flaking to occur. This seems to be derived from the fact in that there exists a difference in greasing contraction factor between the ceramic green sheet 114 and the bonding layer paste 116. That is, when firing the unburned laminate body, the unburned laminate body is degreased during temperature rising step. When this takes place, since the bonding layer paste 116 contains a larger amount of binder or solvent than those contained in the ceramic green sheet 114 in normal practice. Therefore, during temperature rising step, a stress occurs in an area between the ceramic green sheet 114 and the bonding layer paste 116 due to a difference in degreasing contraction as shown in FIG. 17, causing a risk to arise with the occurrence of cracking 99 or flaking.
After the ceramic laminate body has reached the maximum temperature during firing step, the ceramic laminate body is subjected to cooling step. During such cooling step, a stress occurs due to a difference in linear coefficients of expansion of the two ceramic green sheets 112A, 114A, causing cracking 99 or flaking to occur at the same time when the temperature rises. That is, one of the two ceramic green sheets 112, 114 is made of alumina in major proportions and the other is made of zirconia in major proportions. In such a case, the two ceramic green sheets 112, 114 have linear coefficients of expansion different from each other. In such a case, the two ceramic green sheets 112, 114 are hardened in different contracting factors during cooling step in a manner as shown in FIG. 18. In FIG. 18, a curve N1 represents a contracting factor of the ceramic green sheet composed of alumina in major proportions and a curve N2 represents a contracting factor of the ceramic green sheet composed of zirconia in major proportions.
With such a difference in contracting factors, a stress occurs in an area between the two ceramic green sheets, causing cracking 99 or flaking to occur in the sheet 114.